BA does not want to convert all the bmi slots into Longhaul - around 70% of bmi slots will stay as shorthaul over the next few years, so the guys who think that there will be a glut of 319's available to LGW are not necessarily correct.
Who knows what BA will do with LGW, certainly no-one outside BA management know, but I would not be surprised if LGW came to LHR, its all about which airport the most profit can be made.
With so many variables, who knows! All i know, is when BA does makes a decision, it seems a very obvious thing to have done